Women in labour are being refused epidurals, the Department of Health and Social Care has found.
Official guidelines say all women should have the option, but some claim that stretched resources and a lack of information mean it is being denied.
One woman said her "traumatic" experience had left her with post-natal depression and anxiety.
The Royal College of Midwives said "every woman who wants an epidural should be given one if it is safe".
The decision to investigate came after Health Secretary Matt Hancock said in January he wanted "all expectant mothers to be able to make an informed choice that's right for them, to know this choice will be fully respected and to have the freedom to change their mind".
The health minister Nadine Dorries will now write to all heads and directors of midwifery, and to NHS trusts, to ensure guidelines on pain relief are being followed.
These state that women in labour can ask for epidurals at any time, including during the early stage of labour.
But one woman, Angela Spiridis, told the BBC's Victoria Derbyshire programme she was "outright refused" an epidural, when - six hours into the induction - she was tired and in pain.
"At one point I was arguing with four medical professionals, one being the midwife. And they said, 'No, you're not in labour'."
She said she felt "very disempowered".
"They didn't trust me as a woman to know my own body," she added, saying she "felt I was being judged, asking for an epidural".
She said she was then left in the labour room "for several hours", at which point there was no time for an epidural to be administered.
The Royal College of Midwives said its members and services were "focused on ensuring women have the best possible experience of pregnancy and birth".
But it added: "Unfortunately, due to stretched resources, anaesthetists are not always available, which poses real challenges for midwives seeking the best experience for women in labour."
One woman who contacted the Victoria Derbyshire programme - and did not wish to be named - said she had requested an epidural "from the moment contractions began... but was told repeatedly there was no-one to administer it. This went on for three hours".
She had previously experienced post-natal depression with her first baby, and said being unable to have an epidural caused her great anxiety.
"I went on to have post-natal depression, post-partum anxiety and post-traumatic stress," she said.
A Care Quality Commission survey, published in January, found that epidural use in England has increased over the past three years from 28% to 31%.
But the British Pregnancy Advisory Service (BPAS) said women being refused epidurals had become a "common theme".
It said it was leaving women "profoundly traumatised" - with some choosing to "limit" their families as a consequence, and not have another child.
Clare Murphy, from the charity, said a greater emphasis needed to be placed on providing information about pain relief to women.
One mother, Kim McAllister, told the BBC that during her first pregnancy she had "screamed" for an epidural.
"The midwife said, 'No, you're too far gone, keep going'. And that was the end of it - there was no information, no discussion with my husband."
Women who choose to give birth at home or in a midwife-led unit may have to be transferred if they want an epidural, and Ms McAllister said she now understood her request may have come too late.
But, she added, "it was just really scary to be dismissed like that. I was made to feel powerless, at a time when you feel so vulnerable."